Value documents are understood here to be sheet-shaped objects that represent for example a monetary value or an authorization and are hence not to be producible at will by unauthorized persons. Hence, they have features that are not easily produced, in particular copied, whose presence is an indication of authenticity, i.e. production by an authorized body. Important examples of such value documents are chip cards, coupons, vouchers, checks and in particular bank notes.
Such value documents are in many cases to be examined by machine for their authenticity and/or their physical condition, for example, limpness or the presence of tears, holes or adhesive strips.
For such a check there can be used in known apparatuses for processing value documents in particular ultrasound. The corresponding apparatus then has an ultrasonic sensor which can comprise in particular an apparatus for emitting and/or receiving ultrasound. In such examinations, ultrasound can either be emitted continuously or in pulsed fashion onto a value document and the ultrasound thrown back by the value document or preferably the ultrasound passed through the value document be detected; then the corresponding detection signals are evaluated.
Such an ultrasonic sensor can also be used in value document processing apparatuses wherein stacks of value documents are singled, for determining singling errors, in particular the substantially simultaneous removal of two at least partly superposed value documents, also referred to as double or multiple removal.
If the principal direction of propagation or the mean direction of propagation of the ultrasound extends at least approximately orthogonal to the surface of the value document, however, there can occur the undesirable effect that the value document throws the ultrasound back in the direction of an ultrasonic converter used in the ultrasonic sensor. Upon use of continuously emitted ultrasound this, and optionally reflection on the ultrasonic sensor, can result in standing waves which falsify the examination. If the ultrasound is emitted only in the form of pulses, a pulse can be reflected as a so-called pulse echo and likewise impair the examination.